Iceland sits astride the Mid-Atlantic Ridge, part of the world-circling undersea mountain system that is the locus of new crust formation.
Iceland formed by the coincidence of the spreading boundary of the North American and European plates and a hotspot or mantle plume. As the plates moved apart, excessive eruptions of lava constructed volcanoes and filled rift valleys. Subsequent movement rifted these later lava fields, causing long, linear valleys bounded by parallel faults. The divergence of the ridge started in the north about 150 million years ago and 90 million years ago in the south. These movements continue today, accompanied by earthquakes, reactivation of old volcanoes, and creation of new ones. Iceland is the largest island on the ridge because of the additional volcanism caused by the hot spot under the country, which moves slowly towards the northwest across it. Other islands of the Atlantic Ocean created by the volcanism of the Mid Atlantic Ridge are The Azores, Bermuda, Madeira, The Canary Islands, Ascension, St. Helena and Tristan da Cunha.
Because Iceland lies on the Mid Atlantic Ridge, it is being split by the movements of the North American Plate and the Eurasian Plate. The tectonical plates move apart, towards east and west, and both the North American- and Eurasian systems move to the northwest across the hot spot. On top of hot spots is generally a 20-100% molten layer at the depth of 5-20 km, which supplies sufficient material for eruptions. Iceland is home to more than 100 volcanoes, over 25 of which have erupted in recent history. The volcanism on Iceland is attributed to the combination of Mid Atlantic Ridge activity and hot spot activity. Eruptions occur about every 5-10 years and primarily consist of basaltic lava and tephra. A few long-lived centers, such as the volcano Hekla, erupt more silicic magmas. The hot spot causes eruptions within the southern volcanic zone including volcanic systems such as Mt Hekla, the Westman Islands, Katla Caldera, Eyjafjallajokull, the Laki area and the western sub-glacial part of the Vatnajokull area where Grimsvotn are most active. Some of the most active areas of new crust formation are in the southwestern parts of Iceland, accessible to tourists. The trip from Keflavik airport to Reykjavik takes you along the edge of the North American plate where it meets the European plate. A drive to nearby Thingvellir valley, the site of the world's first parliament, reveals an older part of the rift system, where you can see both sides of the plate boundary in one sweeping panorama. A flight to the island of Heimaey gives you a glimpse of new land forming and of the hazards of living in the path of a propagating rift.
Iceland can be divided into three zones based on the age of the basaltic rocks. Tertiary flood basalts make up most of the northwest quadrant of the island. This stack of lava flows is at least 3,000 m thick. Quaternary flood basalts and hyaloclastites are exposed in the central, southwest and east parts of the island. The Quaternary rocks are cut by the neovolcanic zone, areas of active rifting that contain most of the active volcanoes. The rifts are topographic depression bordered by and containing many faults. Fissure swarms make up most of the neovolcanic zone. The swarms are 5-10 km wide and 30-100 km long. The rift zones have opened about 30 m in the last 3,000-5,000 years. The neovolcanic zone is about one-third of the area of Iceland. Almost 60% of the world's regional fissure eruptions have been in Iceland.
Iceland is one of the most active volcanic regions on Earth. It is estimated that 1/3 of the lava erupted since 1500 A.D. was produced in Iceland. Iceland has 35 volcanoes that have erupted in the last 10,000 years. On average, a volcano erupts about every 5 years. Eleven volcanoes have erupted between 1900 and 1998: Krafla, Askja, Grimsvotn, Loki-Fogrufjoll, Bardarbunga, Kverkfjoll, Esjufjoll, Hekla, Katla, Surtsey, and Heimaey. Most of the eruptions were from fissures or shield volcanoes and involve the effusion of basaltic lava.
Iceland was buried under ice in the last Ice Age and all eruptions were subglacial. Remnants of the ice caps remain and Iceland continues to have numerous subglacial eruptions. Of the world's known subglacial eruptions, 83% are in Iceland. The most recent eruption, at Grimsvotn, is an example. Subglacial eruptions produce a special type of volcano, called a table mountain or a moberg mountain. Great volumes of meltwater, generated by subglacial eruptions, can burst out from beneath glaciers to produce enormous floods called jokulhlaup. The discharge can be as much as 20 times greater than the flow rate of the Amazon River.
The 1783 eruption at Laki was the largest single historic eruption of basaltic lava (12 cubic km). Recent eruptions include the 1974-1984 eruption at Krafla, a brief eruption at Hekla in 1991 and again on February 26 2000 and three eruptions at Grimsvotn, in 1996, 1998 and 2004